Fish of the Amazonian IgapÔ: stability and conservation in a high diversity‐low biomass system

Water depth in the Amazon basin can differ by more than I I m between low and high water periods in the annual cycle. When the water is high, the forest streams inundate the surrounding low-lying forest creating vast lakes. Where the water originates from the forest it is rich in humic acids and low in dissolved nutrients. Such waters are termed blackwaters. Igapo is the specialized inundation forest of blackwater regions. Until recently it was generally held that the blackwater fauna was impoverished and of low biomass. However, recent studies have shown that these waters hold a specialized fauna living within and upon allochthonous input from the forest. Within submerged litter banks a diverse ichthyofauna forming dense local populations has been found. Information on the spatial organization, population density and food web of this fauna will be presented. It is argued that this community is highly vulnerable to deforestation and its long-term future can only be assured if the forest is conserved. This is not to suggest that these fish are not adaptable to disturbance as the community has evolved to live with changing water levels and the loss ofsubmerged wood and leaf-litter habitat. It is a remarkable example of a climax community exhibiting high diversity and low total population sizes within a variable habitat. Its conservation is of economic importance to the local human population whose diet includes a high proportion of fish. In terms of number of species the Amazon basin is the richest habitat for freshwater fish in the world. The seasonal pattern of rainfall results in the annual inundation of large areas of forest, producing habitats in which forest and river ecosystems are united. Using as an example a fish community found within a small blackwater forest river, this paper examines the intimate inter-relationship between the forest and the fish which has nurtured and sustained this speciation and adaptive radiation. It is the relationship between the fish and the plants and their products that must be understood if a viable conservation strategy is to be developed. There are two major water types within the Amazon basin. Whitewaters derive their name from the high suspended mineral content and are more nutrient-rich than blackwaters which carry little suspended matter and derive their colour from humic acids. Typical blackwaters are acid (pH 3.8-5-0) whereas whitewaters are approximately neutral. However, clear waters and rivers with intermediate chemical and physical properties do occur. While each water type holds a different fauna, many fish migrate between white- and blackwaters and both are important habitats for fish. In this account, a specialized blackwater leaf-litter community is used as an example of a typical neotropical forest river community. The same general conservation principles that are considered herein would apply to other habitats richer in species and biomass.